1. Field of the Invention
The present invention generally relates to generating pseudo random optical pulse trains, and more specifically to a passively mode-locked laser and method for generating pseudo random optical pulse trains.
2. Description of the Related Art
Target detection systems such as LAser raDAR (LADAR) transmit pseudo random optical pulse trains to track moving targets. Pseudo random optical pulse trains are presently generated by actively modulating each period of a CW (continuous wave) laser with a pseudo random noise (PN) code. The PN code is a random binary code that can be obtained by quantizing the output of a random noise generator. The CW laser is actively modulated by modulating its input power or electrooptically shuttering the light inside the laser cavity in accordance with the PN code. These systems are typically complicated, heavy and expensive.
Lee and Ramaswami, "Study of Pseudo Noise CW diode laser for ranging applications," Proceedings of the SPIE: Cooperative Intelligent Robotics in Space lll, Vol. 1829, 1992, pp. 36-45 disclose a PN modulated CW diode laser for generating pseudo random optical pulse trains. Takeuchi et al, "Random modulation cw lidar," Applied Optics, Vol. 22, No. 9, May 1, 1983, pp. 1382-1386 disclose an electro-optical modulator that modulates the input power of a diode laser with a binary code to produce a pseudo random pulse train. The amplitude modulation can be superimposed on top of a frequency modulation to detect both the range and velocity information of a target without ambiguity. In both the Lee and Takeuchi systems, the pulse train is generated by actively modulating a laser diode. In these systems, the bandwidth of the laser diode output is too broad for many applications.
O'Shea et al, "Introduction to Lasers and Their Applications," Addison-Wesley, 1978, pp. 120-123 disclose a mode-locked laser for producing high-power, short-duration regularly spaced pulses. In a CW laser, the phases of the modes in the laser cavity fluctuate randomly so that the laser generates a fairly uniform output. In the mode-locked laser, the laser is modulated, either actively or passively, to force the modes to have substantially the same phase, i.e. mode-locked. The mode-locked modes form a Fourier sequence such that the superposition of the modes produces a single optical pulse during each laser period. The resulting pulse train is comprised of regularly spaced pulses that are separated by the period of the laser. The period of the laser is defined as the round trip time for light in the laser cavity and is proportional to the length of the cavity. The laser period and time between pulses increase as the length of the cavity increases. The pulse width is determined by the number of lasing modes and is reduced as the number of modes, i.e., the frequency content, is increased. LADAR systems that transmit regularly spaced pulses can either resolve the target's range or velocity; closer spaced pulses improve velocity resolution but reduce range resolution and vice versa.